must I full length size new brass?

New cases are made to be loaded and shot without sizing being necessary. That being said, most could use some neck work. If they are bent or dinged they can be straightened. Sometimes the bullet grip or ID is tighter than you want.

But to talk about FL sizing is really a misconception. The new cases are smaller than your chamber in all dimensions (at least hopefully and theoretically). So running them into your die will not size the case body nor can the die be adjusted down far enough to push the shoulder back. So neck sizing is all you can accomplish anyway.

Personally I will neck size all new cases with a Lee Collet die with a mandrel that is the size I want, outside neck turn to the neck thickness I want, debur the inside of the flash hole, dry lube the inside of the necks and chamfer the neck mouth inside and out. Other than that I don't do any prep.

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I always neck size new brass. I have found a lot of new brass has deformed necks i also trim all new brass to uniform lengths after neck sizing. I also chamfer and debur. Other than that i have really good accuracy out of new brass. I have however found the best accuracy in my rifles with once fired and neck size only. I only full length size brass that is fired in something other than bolt action or if the brass starts to get tight in the chamber. I also full length size if using in more than one rifle of same caliber as stated earlier. I have also gotten really good case life in the neck size only cases vs full length size. These are my processes and arent the only way to do it but they work well for me.

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I just bought new Nosler, and Remington cases for my STW. For the nosler the prep was to chamfer the inside of the case mouth and load them. For the Remingtons I neck sized (numerous ones had digs aroud the mouth), reamed the flash hole, sorted by weight, champer the inside of the neck and loaded. I really like the Nosler case but its a lot of money and I am going to determine which case gives me the best accuracy in my rifle.

US military rifle teams shooting 30 caliber magnums in 1000 yard matches will disagree with this. The shot their best scores using new .300 Win. Mag. cases or .338 Win. Mag. ones necked down to .30-.338. And one several-hundred round batch of ammo did very well indeed across a dozen or more rifles. Civilians did the same thing.

My best 15 to 30 shot test groups with my .30-.338's at a thousand were smallest with either new cases or fired ones full length sized properly. They're equal or smaller than current benchrest records. I and others never got best accuracy with neck sizing previously fired cases.

They can disagree all they wish...but it doesnt change FACTS! Once the round is fired it will force itself to conform to that ONE SINGLE CHAMBER ! All thats reqd after that is neck sizing for proper grip on the bullet ( and of course totla length) if attempted to fire it in ANOTHER CHAMBER...you may certainly run into prolems